1. Field of the Invention
The present invention relates to a vehicular drive system including a differential mechanism having a differential function, and electric motors, and more particularly to techniques for reducing the size of the electric motors.
2. Discussion of Prior Art
There is known a drive system for a vehicle, which includes a differential mechanism operable to distribute an output of an engine to a first electric motor and its output shaft, and a second electric motor disposed between the output shaft of the differential mechanism and a drive wheel of the vehicle. Patent Document 1 discloses an example of such a vehicular drive system, which is a hybrid vehicle drive system. In this hybrid vehicle drive system, the differential mechanism is constituted by a planetary gear set, for example, and a major portion of a drive force of the engine is mechanically transmitted to the drive wheels through the differential function of the differential mechanism, while the rest of the drive force is electrically transmitted from the first electric motor to the second electric motor, through an electric path therebetween, so that the differential mechanism functions as a transmission the speed ratio of which is continuously variable, for example, as an electrically controlled continuously variable transmission, thereby making it possible to drive the vehicle under the control of a control device, with the engine kept in an optimum operating state with an improved fuel economy.
Patent Document 1 JP-2003-301731A
Patent Document 2 JP-3409698B2
Generally, a continuously variable transmission is known as a transmission which permits an improved fuel economy of the vehicle, while on the other hand a gear type transmission such as a step-variable automatic transmission is known as a transmission having a high power transmitting efficiency. However, there is not available any power transmitting mechanism having the advantages of those two types of transmission. For example, the hybrid vehicle drive system disclosed in the Patent Document 1 includes the electric path for transmitting an electric energy from the first electric motor to the second electric motor, namely, a power transmitting path for transmitting a portion of the vehicle drive force as an electric energy, so that a portion of the output of the engine is once converted into an electric energy which is subsequently converted into a mechanical energy to be transmitted to the drive wheels, whereby the fuel economy of the vehicle may possibly be deteriorated under some running condition of the vehicle, for instance, during a high-speed running of the vehicle. Where the above-described differential mechanism is a transmission the speed ratio of which is electrically variable, for example, a continuously variable transmission so-called an “electric CVT”, the vehicular drive system suffers from a similar problem.
As the above-described hybrid vehicle drive system, there is also well known a drive system including a transmission disposed in a power transmitting path between an output member of the differential mechanism (electrically controlled continuously variable transmission) and the drive wheels, for example, as disclosed in the Patent Document 2. In such a hybrid vehicle drive system, a step-variable transmission is disposed in the power transmitting path between the output member of the differential mechanism and the drive wheels, for the purpose of permitting reduction in the size of the second electric motor, by reducing a capacity of the second electric motor that is required upon request of generation of a high drive torque.
In general, a user of the vehicle depresses an accelerator pedal largely and rapidly, when the user wishes to accelerate the vehicle such that a vehicle running speed is increased to a desired speed in a short length of time. In this instance, the length of time required for the increase of the running speed to the desired value is dependent on a gear ratio of the transmission and an engine torque. For example, the required length of time for the acceleration tends to be reduced with an increase of the gear ratio that leads to an increase of the drive torque. It is therefore considered that the acceleration performance can be improved by increasing the gear ratio of the transmission. On the other hand, since the engine rotational speed rises more rapidly as the gear ratio is larger, a shift-up action of the step-variable transmission takes place in an earlier stage. Thus, there is a possibility, depending on a value of the desired speed, that the shifting action could take place a larger number of times until the running speed is increased to the desired speed. Therefore, since the acceleration is temporarily stagnant during a certain length of time that is required by the shifting action, there is a possibility, depending on the value of the desired speed, that the required length of time for the acceleration would be increased with an increase of the gear ratio.
The increased gear ratio leads to a reduction in the acceleration time and the consequent improvement in the acceleration performance, if the shift-up action does not take place until the running speed is increased to the desired speed, or if the number of times of the shift-up action (taking place until the running speed is increased to the desired speed) is not increased in spite of the increase of the gear ratio. However, the increased gear ratio does not necessarily lead to the reduction in the acceleration time and the improvement in the acceleration performance, if the shift-up action takes place a larger number of times until the running speed is increased to the desired speed, since the acceleration is temporarily suspended during the length of time required by the shifting action. Where the acceleration pedal is depressed largely and rapidly, too, the engine rotational speed rises more quickly and the sift-up action takes place in an earlier stage, so that the. acceleration time is not necessarily reduced and the acceleration performance is not necessarily improved, if the number of times of the shift-up action (taking place until the running speed is increased to the desired speed) is increased.
Described in another point of view, it might be possible to improve the acceleration performance, by reducing the length of time required for the shift-up action. It is considered that, unlike the increase of the gear ratio, the reduction of the shifting time necessarily leads to a reduction in the acceleration time and the consequent improvement in the acceleration performance. However, since it is common that the shifting time of the transmission is determined in view of a shifting shock and other factor, the reduction of the shifting time is considered to be difficult. For example, in a step-variable transmission which is shifted by releasing and engaging actions of coupling devices, hydraulic pressures of the coupling devices to be released and engaged for the shifting action are determined depending on the engine torque, so as to reduce the shifting shock and shorten the required shifting time. Therefore, there is a possibility that the shifting shock could be increased if the pressures were determined to shorten the shifting time.
Where the transmission is disposed in the power transmitting path between the output member of the differential mechanism and the drive wheels as in the hybrid vehicle drive system disclosed in the Patent Document 2 for solving problem encountered in the hybrid vehicle drive system disclosed in the Patent Document 1, too, an increased gear ratio does not necessarily lead to the reduction in the acceleration time and the improvement in the acceleration performance, depending on a value of the desired speed, since the acceleration is temporarily suspended during the length of time required by the shift-up action. Further, the reduction of the shifting time is considered to be difficult.
The present invention was made in view of the background art described above. It is therefore an object of this invention to provide a control apparatus for a vehicular drive system including a differential mechanism operable to distribute an output of an engine to a first electric motor and its output shaft, a second electric motor disposed in a power transmitting path between the differential mechanism and a drive wheel of a vehicle, and a transmission constituting a part of the power transmitting path, which control apparatus permits size reduction or improvement of fuel economy of the vehicular drive system, and reduction of a length of time required by a shift-up action of the transmission.